Method of melting steel

ABSTRACT

A METHOD FOR CONTROLLING WITHIN PREDETERMINED LIMITS THE FINAL MECHANICAL PROPERTIES OF STEEL PLATES, BARS AND SHAPES, AT THE MELTING STAGE OF SAID STEEL BY EQUATING THE CHEMISTRY THEREOF TO THE TENSILE STRENGTH BY MEANS OF THE FORMULA:   WHERE THE DIVISORS ARE PREFERABLY X=6, Y=5, AND Z=15, BUT MAY VARY BETWEEN ABOUT, X=4.5 TO 7.5, Y=4 TO 6, AND Z=12 TO 18, SO THAT THE A.E.F. IS CHANGED TO FALL WITHIN A LIMITED RANGE OF A CALCULATEED AIM A.E.F. FOR SAID TENSILE STRENGTH.

Aug. 8, 1972 v H. w. BENNETT METHOD OF MELTING STEEL Filed June 1, 1970INVENTOR/S ms \zsc ax 15255 362B 2% 590m $5 8 R93 E 33 8 o 9 om Om 9 E33HE H -Eo N Om mk dl :wwas mkqdl r Om HOWARD W BENNETT l/l/fl,

ATTORNEYS United States Patent 3,682,711 Patented Aug. 8, 1972 US. Cl.148-2 5 Claims ABSTRACT OF THE DISCLOSURE A method for controllingwithin predetermined limits the final mechanical properties of steelplates, bars and shapes, at the melting stage of said steel by equatingthe chemistry thereof to the tensile strength by means of the formula:

where the divisors are preferably X:6, Y:5, and Z:15, but may varybetween about, X=4.5 to 7.5, Y=4 to 6, and Z=12 to 18, so that the ischanged to fall within a limited range of a calculateed aim A.E.F. forsaid tensile strength.

BACKGROUND OF THE INVENTION This invention relates to a unique method ofcontrolling the final mechanical properties of carbon-bearing steelplates, bars and shapes at a very early stage in processing, i.e. themelting stage.

It will be appreciated that where a minimum of processing steps areapplied to a steel to afiect the metallurgical properties thereof,advantage must be taken during the few opportunities presented. Whilethe chemistry of the steel represents a prime situation to eifectproperty control, heretofore such an accurate system has not been known.

But before reviewing the prior art attempts to control final properties,it will be recalled that carbon steel plate for example, such as ASTMA572 Grade 42, is sold primarily based on minimum strength levels.Experience has shown that even where the chemistry is withinspecification limits, mechanical properties may vary from below theminimum to well in excess thereof. The latter is also an undesirablesituation as too much strength may present fabricating difficulties.

And, since such steel plates as noted above are frequently sold in thehot reduced condition, i.e. without property modifying heat treatments,the prior art practitioners attempt to limit property variations solelyby specifying chemical composition. In addition, it was necessary toestablish separate specifications for different thicknesses for eachgrade or strength of steel. The method of melting, whether by openhearth, BOF or electric furnace, may also be a factor in writing thespecifications. But even with such elaborate measures, heats were missedthereby giving rise to abnormally large material and processing costs.

Up until the present time no one was fully cognizant of an inexpensive,yet accurate way of controlling final mechanical properties at themelting stage. Such steels were normally made by melting within aspecified chemistry rangeeach controllable element such as carbon,manganese and silicon having its own range. Little attention was paid toresidual elements such as chromium, molybdenum, nickel and copper, andthe cumulative effect was not considered. Thus, even when a steel metthe chemistry limits, it was possible for the mechanical properties tobe out of range because of all elements being high in their range orvice versa. With the current development, a system has been found topredict and/or control the final mechanical properties of carbon steelplates, bars and shapes through a correlation of the effects of residualelements and intentional alloy additions on the properties.

SUMMARY OF THE INVENTION The present development is based upon a methodof utilizing and predicting the effect of such residual elements aschromium, molybdenum, nickel, copper, as well as the additions such asvanadium, columbium and manganese on the final mechanical properties ofcarbon bearing steel plate. By the use of an alloy equivalent formula,hereinafter referred to as A.E.F., in melting heats of steel, advantagecan be taken of the melt-in residuals. This permits much better controlof final properties. Also, cost reduction can be made by using lesseramounts of elements than would normally be made.

In the melting of the steel, whose chemistry by weight comes within thefollowing:

Percent maximum Carbon 0.60

Manganese 2.00 Chromium and molybdenum 0.75 Nickel and copper 0.75Columbium 0.06

Vanadium 0.15

Silicon 1.00 Iron Balance, except for normal residuals and additivessuch as boron, titanium, zirconium, etc., a preliminary determination ofthe chemistry is made prior to the casting or tapping from the furnace.Based upon a predetermined A.E.F. as calculated from prior data, or inthe case of steel plate from the accompanying figure, the chemistry isadjusted by varying one or more of the recited elements. Said adjustmentis controlled by the following formula:

X=4.5 to 7.5, preferably equal to 6 Y=4 to 6, preferably equal to 5 2:12to 18, preferably equal to 15.

By following this formula, a heat of molten steel can be quicklyadjusted by varying one or more of the elements included in the formula.This will save time in shaping up a heat prior to casting or tapping,and minimize rejected heats due to large differences between projectedand actual mechanical properties.

BRIEF DESCRIPTION OF THE DRAWING The figure is a graph of aim A.E.F. vs.aim tensile strength of a semi-killed steel plate, with final platethiokness superimposed thereover.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Before discussing thedetails of the preferred embodiment of this invention, it must beacknowledged that metallurgical formulas, to predict performance ofmaterials in service, do exist. Thus, it is not the intention herein toassert that the present invention was the first to recognize a weightedrelationship between chemistry and performance. However, the discoveryis significant in that it is the first to use during melting the totalpercentage of additives and residuals to make adjustments in one or moreelements to achieve the desired strength level.

The formulas which have been developed and applied in the metalfabricating field are empirical and based on actual experiences ofindividual mills, research laboratories, etc. One of the more commonones is the carbon equivalent (C.-E.) employed by fabricators to predictthe weldability of steel. In the case of new steels, or ones with whichthey are not familiar, the fabricator can use the OE. to set up weldingprocedures for the steel. However, this is a measure of the finalproduct and not a control step in producing steel.

Another area where formulas have been applied is in the area ofdetermining hardenability of ferrous products. Specifically, methodshave been devised for calculating the Jominy curve for a given steel.Equipped with this knowledge, a designer can more effectively heat treatthe steel. However, the method is admittedly not precise as the elfectof the elements making up the chemistry is not independent of oneanother throughout the chemical range permitted by the specificationbeing used. As an example, there is an interaction between carbon andcertain of the alloying additions. Further, the effect of a givenaddition is not constant over a broad percentage range. Nevertheless,such a formula is helpful in determining hardenability.

Contrary to some aspects of the preceding, the present inventioncontemplates a system for melting steel for use as plates, bars andshapes, which system permits the manufacturing mill to sell the steelproducts to a tighter mechanical property range. The system also meanslower costs as the melting operator can take advantage of the melt-inresiduals, and control the final properties at the desired levels.

The system herein is based upon the recognition each of the elementscarbon, manganese, chromium, molybdenum, nickel, copper, vanadium andcolumbium, has on developing the strength in carbon bearing steels. Bythe formula to be given hereinafter, the system in very simple termsprorates the other elements contained in the steel with respect to thepercent carbon, based on the effects of each element content onmecham'cal properties. The formula for the system is as follows:

Note.Formula valid for steels whose chemistry by weight comes within thefollowing:

residuals and additives such as boron, titanium, zirconium, etc. Whilethe formula is valid for those steels falling within the abovechemistry, it can be simplified to the extent of semi-killed steels vs.silicon killed steels. In the former, only a slight amount is present,i.e. up to about .10%. On the other hand, silicon killed steels maycontain as much as .3 Thus, the formula may be simplified to the extentof deleting the silicon factor in determining tensile strength ofsemi-killed steels. In the preferred formula, the divisors are asfollows: X='6, Y=5, and Z=15. However, it is recognized that variationsin mill practices, product shapes, alloys and other factors will differfrom one manufacturing mill to another.

Therefore, this invention contemplates an expanded range for the severaldivisors as follows:

X=4.5 to 7.5 Y=4.0 to 6.0 Z=l2.0 to 18.0.

One additional factor which warrants discussion is the relationship ofthickness to final mechanical properties. For convenience in helping tounderstand the effect of thickness, reference is made to the figure. Ingeneral, for a given strength level, as the section thickness increases,higher aim A.E.F. values are sought. The figure, which is applicable tosemi-killed steel plates, plots aim .AIE.F. against final steel platetensile strength, with plate thicknesses superimposed thereover.

The formula and figure are used in the following way. The chemistry(preliminary determination) of the molten steel is incorporated into theformula. Then changes are effected therein so as to bring the A.E.F. tothe level desired, within i.05 and preferably :.02, for the productionof steel plate of a given thickness having the specified mechanicalproperties, more specifically tensile strength, within relatively narrowlimits, i.e. i5 k.s.i. (34.5 MN/mF).

It is pertinent to compare the A. E.-F. formula used in this inventionwith a well known measure of weldability as represented by the formulaby Winterton:

Mn Ni N0te.-As the C.=E. approaches or exceeds .55, the problem ofweldability increases.

This -C.E. formula has obvious similarities but there are some importantdifierences. For example, molybdenum and vanadium are additive factorsin the current formula and negative factors in the CE. formula; they arealso 10 times more significant in their contribution to than in theireflect on C. -E. Colum'bium is equal to carbon in strengthening effectbut does not even appear in the CE. formula.

Up to this point, little has been said about the types or grades ofsteel to which this invention relates, except as to the maximum limitsplaced on the specified elements. In general, the system for meltingsteel as taught herein is directed to those steelswhich arecharacterized as hot rolled steel plates, bars and shapes, and whichvary in size from about to 8" (.48 to 20 cm.). That is, they are usuallysold in the hot rolled condition. The system as proposed is applicableto heat treated ferrous products although a modified curve would berequired for heat treated products. Likewise, the size and shape of themill treated product would affect the shape of the curve. Thus, for hotrolled bars and shapes, the curve would vary slightly from the figure,which is adapted for use with plates.

It should be apparent from the A.E.F. formula herein that the carbon isconsiderably more eifective than the residual elements. Nevertheless,they do, exert an influence on final mechanical properties.Unfortunately, the precise influence was not recognized such thatreliance was placed solely on the carbon content. As a result, heatswere lost and/or subsequently diverted for excessive mechanicalproperties. With the present invention, such problems can be avoided.

For convenience, but without desiring to so restrict the invention,consider the preferred A.'E.F. formula and the accompanying figure inthe following illustration.

Under typical mill operating conditions, a melter is provided with thenecessary data concerning the final mill product, i.e. type of steel orchemistry limits, mechanical property limits, and mill product size,among other detainls of processing. Therefore, for this illustration,assume the following data:

(a) Type steel-semi-killed carbon steel plate having a final chemistryfalling within Carbon .35 maximum. Manganese .85-1.20%. Phosphorus .025%maximum. Sulfur 060% maximum. Silicon .10% maximum. Copper .60% maximum.Nickel .60% maximum. Chromium .25% maximum. Molybdenum .25% maximum.Iron Balance.

(b) Tensile strength75/ 85 k.s.i. (517/586 MN/m?) (c) Mill productHRsteel plate /2" thick (d) Aim A.E.F.=.47

Percent Copper .25 Nickel .20 Chromium .15 Molybdenum .10

Since copper, nickel and molybdenum are not oxidizible and will notchange during the refinement of the heat, and chromium will change onlyslightly through the action of oxygen, a portion of the A.E.F. formulawill be fixed. Thus, the balance needed to bring the A.E.F. value up tothe aim A.E.F. may be readily determined by simple arithmetic. Inutilizing the preferred formula taught herein, a substitution of theabove residuals will yield an A.E.F.=.08.

- To bring the A.E.F. up to the desired aim value, the melter must addone or more elements, preferably carbon and manganese, to effect theadjustment. However, by using the formula herein, the melter isrelatively free to select the amount, type and number of elements to beadded.

Just prior to tapping the heat, a final chemistry may be taken to detectany changes in the residual levels, and to determine the carbon andmanganese contents. In this illustration, it would not be unusual tofind about .12% manganese, and about .10% carbon in said tap chemistry.By adjusting the two elements, whose final content will provide themissing factors in the A.E.F. formula, the aim A.E.F. will be reachedthereby assuring the desired tensile strength within the specificationrange of 75/85 k.s.i. (517/586 MN/m. For example, if manganese isincreased to about 1.00%, the A.E.F. from residuals will be. raised from.08 to about .25. To reach the aim A.E.F. of .47, 27 points of carbon,or .27% is needed (an increase of about .17% over tap chemistry).

By following this formula, and anticipating normal elemental recoveriesconsistent with present steel making practice, one can provide a millproduct having the desired strength within :5 k.s.i. (34.5 MN/m. Forinstance, while 1.00% is the aim for manganese, it could vary within thespecification range, thereby changing the final A.E.F. The final carbonis more predictable even though some variation from aim is to beexpected. Nevertheless, even with such variation, predictable resultscan be secured. That is, a final actual A.E.F. within :05 of aim A.E.F.,will yield a mill product having a tensile strength within thespecification range of 75/ k.s.i. (517/586 M-N/m.

While the above illustration has been simplified for the purpose ofproviding an exemplary embodiment, it should be apparent variouscombinations of changes may be effected by the melter to achieve thedesired results. For instance, vanadium and/or columbium may be addedwith a concurrent reduction in carbon. On the other hand, whereresiduals are low, additions may be made with full knowledge on theireffectiveness in increasing the strength of the final steel product.

Accordingly, since modifications are contemplated, particularly by thoseskilled in the art after reading these specifications, no limitation isintended to be imposed herein except as set forth in the appendedclaims.

What is claimed is:

1. In a method for controlling the final tensile strength of ferrousmill products within 134.5 MN/rn. of a predetermined value at themelting stage thereof, wherein a cast ferrous body is rolled into a millproduct having a thickness between about 0.48 cm. to 20 cm. and saidproduct is not given any subsequent property modifying heat treatmentsand where the final chemistry falls essentially by weight within thefollowing limits;

Percent maximum Carbon 0.6

Manganese 2.00 Silicon 1.00 Chromium and molybdenum 0.75 Nickel andcopper 0.75 Columbium 0.06

Vanadium 0.15 Iron Balance where the divisors may vary between about,X=4.5 to 7.5, Y=4 to 6, Z=l2 to 18, so that the A.E.F. is changed tofall within a range of :005 of a calculated aim A.E.F. for saidpredetermined value of tensile strength.

2. The method claimed in claim 1 wherein said divisors are X=6, Y=5, andZ: 15.

3. The method claimed in claim 1 wherein said thickness varies betweenabout .64 to 5.08 cm.

4. The method claimed in claim 3 wherein said ferrous mill product is asemi-killed carbon steel Whose final thickness varies between about .64to 2.54 cm., and that said aim A.E.F. is determined from theaccompanying figure.

5. The method claimed in claim 1 wherein said predetermined value variesbetween about 414 to 620 MN/ m? for said mill products in the hotreduced condition.

References Cited UNITED STATES PATENTS 1,838,425 12/1931 Marsh 75129X3,310,441 3/1967 Mandich l4 836 OTHER REFERENCES Quest, C. F., and T. S.Washbum: Tensile Strength and Composition of Hot-rolled Plain CarbonSteels, Trans. AIME, 1940, v. 140, pp. 489-496.

Mottley, Charles M.: The Application of Statistical Methods to theDevelopment and Quality Control of High Tensile Steel, 1. Am. Soc. NavalEngrs., 1945, v. 57, pp. 21-55.

Mudd Series, Seeley W.: Basic Open Hearth Steelmaking, A.I.M.M.E., NewYork, 1951, pp. 500-504.

F. B. Pickering and T. Gladman: BISRA conference on carbon steels,Harrogate, May 1963, pp. 10-25.

K. J. Irvine and F. B. Pickering: Low-carbon steels L. DEWAYNE RUTLEDGE,Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R. 75-129;148-36

